Japanese monkeys were trained in a symbolic construction matching-to-sample (SCMS) task in which 6 compound figures symbolically represent 6 objects. Behavioral experiments showed that the monkeys could learn the SCMS task, and generalize from item-object associations to category-object associations. We developed ultra-thin flexible electrode arrays for electrocorticogram (ECoG), and implanted them into the prefrontal cortex of the monkeys. During the SCMS task, ECoG responses to the second sample element was induced earlier and became larger than during sample presentation of a compound figure in a symbolic matching-to-sample task.